(1) Field of the Invention
The present invention relates to a support sheet for a thermal transfer image-receiving sheet and a method of producing same. More particularly, the present invention relates to a support sheet useful for thermal transfer full colored image-receiving sheets, thermal sensitive image-recording sheets, and thermal printing sheets, especially sublimating dye-thermal transfer image-receiving sheets, capable of receiving thermally transferred dye or ink images thereon in a clear and sharp image from without thermal curling thereof, and capable of recording thereon continuous tone full colored images or pictures at a high resolution and a high tone reproduction.
(2) Description of the Related Arts
It is known that new types of color printers, for example, relatively compact thermal record printing systems having a thermal head, enable printing of clear colored images or pictures by thermal transfer of the colored images or pictures of a thermomelting ink or sublimating dye onto an image-receiving sheet. There is great interest in the further development and utilization of these printing systems, especially the sublimating dye colored image or picture-thermal transfer printing systems.
In the operation of the sublimating dye image or picture-thermal transfer printing system, an image-receiving sheet having a polyester resin layer, on which the sublimated dye is easily dyed, is superimposed on an ink sheet comprising a support sheet consisting of a thin plastic sheet and a sublimating dye ink layer formed on a surface of the support sheet, in a manner such that the surface of the polyester resin layer of the image-receiving sheet comes into contact with the surface of the ink layer of the ink sheet, and the ink sheet is partly heated imagewise by a thermal head in accordance with electric signals corresponding to the images or pictures to be printed, to thermally transfer the ink images or pictures composed of the sublimated dye, and having a color density corresponding to the amount of heat applied to the ink sheet on the polyester resin layer of the image-receiving sheet.
It is also known that a support sheet comprising a sheet substrate and a coating layer formed by bonding a bi-axially drawn plastic film consisting of a mixture of an inorganic pigment and a polyolefin resin and having a multilayered structure to the sheet substrate surface enables thermal transfer image-receiving sheets to receive thermally transferred images or pictures having high quality from a printing system having a thermal head.
In the image-receiving sheet for the sublimating dye thermal transfer printing system, the above-mentioned support sheet is coated with a thermal transfer image-receiving layer comprising, as a principal component, a polyester resin.
The record sheet or image-receiving sheet having the above-mentioned support sheet has an even thickness, a high softness, and lower thermal conductivity than that of paper composed of cellulose fibers, and therefore, is because images or pictures having a high uniformity and color density can be formed thereon. Nevertheless, where the coating layer in the support sheet is formed from a bi-axially drawn plastic film comprising, as a principal component, a polyolefin, for example, polypropylene resin, and having a multilayered structure, and ink or dye images or pictures are thermally transferred by heat from a thermal head to the polyester resin coating layer in the image-receiving sheet, the multilayer structured polyolefin resin coating film in the support sheet is heated by the thermal head so that a drawing stress held in the polyolefin resin coating film is released, and thus the polypropylene resin coating film layer shrinks. This shrinkage of the polyolefin resin coating layer causes the image-receiving sheet to be curled and a number of wrinkles to be formed thereon, so that the forwarding of the sheet in the printing system is hindered by the curls or wrinkles on the sheet and the resultant prints have a reduced commercial value.
To eliminate the above-mentioned disadvantages, a new type of support sheet was provided by coating two surfaces of a sheet substrate having a relatively small heat shrinkage with the multilayer-structured plastic coating films. This type of the support sheet effectively prevents the formation of wrinkles on the image-receiving sheet due to the heat shrinkage of the plastic coating films, but since two coating films having different heat shrinkages are laminated on a sheet substrate, the resultant image-receiving sheet is naturally not free from curl-formation. Especially, in the sublimating dye thermal transfer printing system, a large quantity of heat is applied to the image-receiving sheet, and therefore, the above-mentioned problems often occur on the image-receiving sheet.
The sublimating dye thermal transfer printing system is a mainstream printing system among small size non-impact type full colored image-printing systems, and thus is often used as a printer for small size electronic cameras or video printers. Therefore, there is an urgent demand for the provision of a new type of support sheet for a thermal transfer image-receiving sheet which can form clear images or pictures thereon without thermal deformation thereof, even when used for the sublimating dye thermal transfer printing system in which a large quantity of heat is applied to the
image-receiving sheet.